Homebrewing, VRX-1

DIY Isolated Jack

The VRX-1 receiver kit uses a TDA7052 audio amplifier IC, which is a bit different from your run-of-the-mill LM386. Because of its push-pull output, if you use a phones jack, you must isolate it from ground. This isn’t something you have to worry about if you use a non-conductive enclosure or if you have an isolated phones jack. However, more often than not, you won’t have an isolated jack, and many of the enclosures that we use are metal.

This little hint will allow you to modify a “standard” grounded ring phones jack with a conductive enclosure (such as the 4SQRP Clear Top Tin, like the one I used).

Bill of Materials

  • 3.5 mm phones jack
  • Small length of 3/8″ OD (1/4″ ID) Polyethylene Tubing
  • 1/4″ nylon washer
  • CA glue (optional)

Instructions

Cut off a piece of the 3/8″ OD polyethylene tubing approximately 1/16″ long (about 1 mm).

Cutting a small piece of the 3/8" OD tubing
Cutting a small piece of the 3/8" OD tubing
Place the small piece of tubing over the phones jack, so that it rests against the body. You may want to glue the tubing to the body of the phones jack.

3/8" OD tubing over jack
3/8" OD tubing over jack
Drill a 3/8″ hole in your enclosure to accommodate the tubing, then place your phones jack in the hole so that the 3/8″ tubing is centered in the hole. Place the 1/4″ nylon washer over the jack from the outside of the enclosure, then use the knurled nut to secure the jack to the enclosure. It’s as simple as that!

Isolated jack installed in enclosure
Isolated jack installed in enclosure
Design, Homebrewing, VRX-1

Introducing the VRX-1

VRX-1 in 4SQRP Clear Top Tin
VRX-1 in 4SQRP Clear Top Tin

While I was away on my honeymoon, I noticed that the upcoming kit that I’ve been hinting about for months has finally been released. The Four State QRP Group announced availability of the VRX-1 direct conversion receiver. The VRX-1 is a simple 40 meter VXO-tuned receiver (crystal on 7.030 MHz), but it’s not your typical NE602/LM386 combo. The product detector consists of only a 2N7000 MOSFET, a capacitor, and an inductor. The audio amplifier is a TDA7052 from NXP. This little 8-pin DIP can output 1 watt of clean audio into low impedance headphones or a small speaker. Current consumption is only about 40 mA, which makes the VRX-1 easy on your batteries if you take it out for portable use. The construction of the receiver is done Manhattan-style, but don’t let that put you off if you’ve never built this way before. I provide a precise, detailed layout diagram to show you exactly where each part is placed and how it is oriented. There’s also some very detailed build documentation to walk you through the build, which you can preview at the VRX-1 web page. Even the novice builder can construct this radio!

The VRX-1 was designed to be a companion to the NS-40, or other similar rock-bound 40 meter QRP transmitters. I also include instructions on how to use some of your own parts to modify the VRX-1 for operation on any HF band, so don’t feel like you are stuck on 40 meters if you would like to try to experiment a little. In a future blog post, I’ll walk you through the process of integrating the VRX-1 with a standalone QRP transmitter to make a complete station.

Proceeds from the kit sales go to fund OzarkCon 2010; I don’t make a dime off of it (just the glory, LOL!). So please support the QRP community and try your hand at a new kind of kit. It’s only $25 postage paid in the States, $28 for DX.